Radiation astronomy/Oort clouds

This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in astronomical units (AU). The scale is logarithmic, with each specified distance ten times further out than the previous one.

An artist's rendering is of the Oort cloud and the Kuiper belt (inset). Sizes of individual objects have been exaggerated for visibility.

The Oort cloud or the Öpik–Oort cloud[1] is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.[2] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.[3]

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Most of the comets lay at the distant reaches of our system in a hypothesized Oort cloud. At the very edge of the solar system, these comets orbit in very large loops around the distant reaches of our solar system. The passing of nearby stars, or other objects can alter their orbit, sending them speeding towards the inner reaches of our solar system. These comets typically retain very large orbits such that they will not return (once seen in the inner solar system) for many thousands of years.

Cosmic "ray protons at energies up to 10 GeV [may be] able to build-up large amount of organic refractory material at depth of several meters in a comet during [its] long life in the Oort cloud (~4.6 x 107 yr). Ion bombardment might also lead to the formation of a substantial stable crust (Johnson et al., 1987)."[4]

Sedna was discovered from an image dated 2003-11-14 at coordinates 03 15 10.09 +05 38 16.5. The 3 overexposed stars are apparent magnitude 13. The "bright star" near Sedna is apmag 14.9 and about the same magnitude as Pluto. (Wikisky image of this region) The picture shows an area of the sky equal to the area covered by a pinhead held at arm's length. Sedna is too faint to be seen by all but the most powerful amateur telescopes.

2015 TG387 (nicknamed The Goblin for the letters TG and because its discovery was near Halloween),[5][6] is a trans-Neptunian object (TNO) and sednoid in the outermost part of the Solar System.[7] It was first observed on October 13, 2015, with the Subaru Telescope at Mauna Kea Observatories, and publicly announced on October 1, 2018.[8][9]

2015 TG387 is the third sednoid to be discovered, following 90377 Sedna and 2012 VP113.[10][11] It is estimated to be 300 km (190 mi) in diameter.[10]

Along with the similar orbits of other distant TNOs, the orbit of 2015 TG387 suggests, but does not prove, the existence of a hypothetical Planet Nine in the outer Solar System.[10][9]

As of 2018, the object is 80 AU from the Sun; about two-and-a-half times farther out than Pluto’s orbit.[6] As with Sedna, it would not have been found had it not been on the inner leg of its long orbit, which suggests that there may be many similar objects, most too distant to be detected by contemporary technological methods, and implies a population of about 2 million Hills cloud, or inner Oort cloud, objects larger than 40 km (25 mi), with a combined total mass of 1022 kg, which is several times the mass of the asteroid belt.[10]